In eukaryotic cells DNA is packaged with histones to form chromatin. Approximately 150 base pairs of DNA are wrapped twice around an octamer of histones (two each of histones 2A, 2B, 3 and 4) to form a nucleosome, the basic unit of chromatin. Changes in the ordered structure of chromatin can lead to alterations in transcription of associated genes. This process is highly controlled because changes in gene expression patterns can profoundly affect fundamental cellular processes such as differentiation, proliferation and apoptosis. Control of changes in chromatin structure (and hence of transcription) is mediated by covalent modifications to histones, most notably of their N-terminal tails. These modifications are often referred to as epigenetic because they can lead to heritable changes in gene expression, but do not affect the sequence of the DNA itself. Covalent modifications (for example, methylation, acetylation, phosphorylation and ubiquitination) of the side chains of amino acids are enzymatically mediated.
The selective addition of methyl groups to specific amino acid sites on histones is controlled by the action of a unique family of enzymes known as histone methyltransferases (HMTs). The level of expression of a particular gene is influenced by the presence or absence of a methyl group at a relevant histone site. The specific effect of a methyl group at a particular histone site persists until the methyl group is removed by a histone demethylase, or until the modified histone is replaced through nucleosome turnover. In a like manner, other enzyme classes can decorate DNA and histones with other chemical species and still other enzymes can remove these species to provide temporal control of gene expression.
The orchestrated collection of biochemical systems behind transcriptional regulation must be tightly controlled in order for cell growth and differentiation to proceed optimally. Disease states result when these controls are disrupted by aberrant expression and/or activity of the enzymes responsible for DNA and histone modification. In human cancers, for example, there is a growing body of evidence to suggest that dysregulated epigenetic enzyme activity contributes to the uncontrolled cell proliferation associated with cancer as well as other cancer-relevant phenotypes such as enhanced cell migration and invasion.
Rearrangements of the mixed lineage leukemia (MLL) gene on chromosome 11q23 are associated with aggressive leukemias with a poor prognosis. MLL translocations result in aberrant recruitment of DOT1L, a histone methyltransferase that methylates lysine 79 of histone H3 (H3K79), to chromatin leading to ectopic H3K79 methylation and increased expression of genes involved in leukemogenesis. These rearrangements, which are found in over 70% of infant leukemias and approximately 10% of adult acute myeloid leukemias (AML), result in the expression of fusion proteins in which the C-terminal sequences of MLL, including a SET-domain that methylates lysine 4 of histone H3 (H3K4), are replaced with sequences derived from a variety of fusion partners, including AF4, AF9, and ENL. The majority of these fusion partners are components of transcriptional elongation complexes that, directly or indirectly, recruit DOT1L to genomic loci bound by the MLL-fusion protein. This results in elevated H3K79 methylation and increased mRNA expression of MLL-fusion target genes, such as HOXA9 and MEIS1 that are central to the pathogenesis of leukemia.
Mistargeted DOT1L enzymatic activity has therefore been proposed as a driver of disease in MLL patients, however in the absence of specific DOT1L methyltransferase inhibitors; this hypothesis has not been directly addressed in model systems.
Beyond cancer, there is growing evidence for a role of epigenetic enzymes in a number of other human diseases, including metabolic diseases (such as diabetes), inflammatory diseases (such as Crohn's disease), neurodegenerative diseases (such as Alzheimer's disease) and cardiovascular diseases. Therefore, selectively modulating the aberrant action of epigenetic enzymes holds great promise for the treatment of a range of diseases.